An illusion is simply a state where the model of the world created by the mind differs from the world as it exists. In a pedantic sense, it’s all an illusion,

An illusion is simply a state where the model of the world created by the mind differs from the world as it exists. In a pedantic sense, it’s all an illusion, but there are particular illusions where the difference is readily demonstrable — when the magician shows what was hidden in his sleeves, as it were. In some sense, most of science is exposing the method of various tricks our own consciousness plays on us. We want to know, for example, why metal appears to be shiny— when the phenomenon we’re studying ‘shininess’ only exists in our own minds. There’s no innate ‘shiny’ property in nature. Time is one of those things where there is a persistent feeling that it must be illusory (which is to say, something created by our minds), but we’re not quite sure exactly in what way or how to prove it.